Mucinous tumors make up 15% to 20% of all primary ovarian tumors. About 80% of those are benign (mucinous cystadenomas) and the remainder 20% consist of borderline tumors and carcinomas. They affect a wide age range with a mean age at diagnosis of 50 years. Symptoms include abdominal pain, distension, pelvic mass, and, in rare cases, hormonal manifestations due to stromal luteinization. Grossly, they are large tumors with a smooth surface, partially or completely cystic, often multiloculated and filled with viscous mucoid material. The lining epithelium can be intestinal type (more common) or endocervical type. The photograph shows an 11.0 cm diameter multilocular mucinous cystadenoma of the ovary that was filled with mucoid fluid. Note the smooth inner surface. The lining was endocervical type. Image courtesy of : Ed Uthman, MD, Houston, Texas. Used with permission.